1. Field of the Invention
This invention relates to low density polyurethane foams and, more particularly, this invention relates to molded foamed polyurethanes that are formulated to better retain certain of their physical properties after exposure to elevated temperatures and pressures.
2. Definitions
The following terms as used in the specification and claims, are defined as follows:
1. Last. A foot-shaped form on which the parts of a shoe are assembled to make a shoe.
2. Upper. The part of a shoe that encases the foot.
3. Outsole. The bottom part of a shoe that makes contact with the ground.
4. Midsole. A cushioning material between the upper and the outsole.
5. Insole. A pad, often removable, on which the foot rests inside the shoe. It is sometimes called the sock liner or the footbed.
6. Sport Shoe. Casual footwear that is generally constructed with a rubber sole bonded, as opposed to sewn, to a cloth (canvas), rubber or leather upper. Examples of casual footwear include slip-ons and sneakers and tennis, basketball, running and deck shoes.
7. Low Density Foam. A foam having a density less than 0.3 g/cc.
8. Compression Set. A measure of a rubbery compound's retention of its elastic properties expressed as the ratio of the loss in thickness to the original thickness of a specimen that is compressed under constant load or constant deflection for a defined period of time at a defined temperature. The values for compression set as given in this specification and claims are based upon ASTM D-3574, test D, constant deflection compression set test using a 50% deflection at 140.degree. C. for one and one half hours.
9. One Shot Molding. A method for making polyurethanes in desired sizes and shapes by reacting a polyol blend and an isocyanate in situ in a mold. It differs from two step methods in which a prepolymer is first prepared.
10. Cold Cure. A molding method in which the mold temperature does not exceed about 60.degree. C.
11. Isocyanate Index. The NCO equivalent divided by the reactive hydrogen equivalent.
12. Polymer Polyol. A composition produced by polymerizing one or more olefinically unsaturated monomers dissolved or dispersed in a polyol in the presence of a free radical catalyst as more fully disclosed, for example, in U.S. Pat. No. 4,208,314 incorporated herein by reference.
13. Vulcanization. A term used in the shoe industry to describe a method of bonding the various components of a sport shoe together by subjecting the components to elevated temperatures and pressures as they are assembled around a last. The term as so used in the shoe industry may or may not accurately reflect the ordinary meaning of vulcanization as used and understood in the rubber industry.
3. Description of the Prior Art
This invention relates to one shot molded polyurethane foamed parts that are suitable for use as components for sport shoe. In the ordinary construction of a sport shoe, there may be included between the outer sole and the upper a number of components such as, for example, innersoles, midsole, and arch supporters. One shot molded foam polyurethanes are well suited materials from which to make these intermediate parts because of a number of desirable properties including flexibility, resilience and low density. The polyurethanes are also useful because they can be formulated to be soft to the touch and comfortable to the foot.
In one method of making sport shoes, the components of the shoe are bonded together with adhesives. By another method of construction, referred to in the trade as vulcanization, the components of the shoe are heated to vulcanization temperatures and the rubber components are cohered, under pressure, to each other. Typically, the vulcanization process is conducted at a temperature of about 140.degree. C. for a period of about one and a half hours. As a general rule, the vulcanization method is preferred to the use of adhesives because it is simpler and cheaper. Vulcanization is also superior to adhesive bonding in that the shoe is more durable and can be washed, for example, in hot detergents without any concern of dissolving or softening an adhesive. One drawback of the vulcanization procedure is, however, that components such as midsoles, insoles and arch supporters cannot be molded from polyurethanes since the one shot, low density, foamed polyurethanes known to the prior art cannot survive vulcanizing temperatures and pressures without a substantial diminution in some of their desired physical properties. Arbitrarily the sport shoe industry has established the criteria that if the components of a sports shoe made by vulcanizing are to be useful, they should have a compression set of less than 20% and, for some applications, also should pass a bend test in which the polyurethane foam is heated in a 140.degree. C. oven for one hour and a half and doubled back on itself (180.degree. bend) without breaking immediately after it is taken out of the oven.
One process for making polyurethanes with a better high temperature property of modulus of torsional elasticity is disclosed in U.S. Pat. No. 4,698,410. This patent makes use of certain urea derivatives reacted with conventional isocyanates and polyols to achieve better high temperature properties and is concerned with dense or foamed polyurethanes with densities above 0.3 g/cc.
It is also known that polyurethanes with improved resistance to thermal degradation can be obtained if polyester polyols are used rather than polyether polyols. However, because polyester polyols are significantly more expensive than polyether polyols and for other reasons, it is preferred to use polyether polyols in making components for sport shoes.